Development, survival and activation of B cells are regulated by signals derived from the surface expressed B cell antigen receptor (BCR). Src-family protein kinases (SFK) are implicated in both negative and positive regulation of BCR signaling. Changes in the activity and expression of SFK have been linked to immunodeficiency, autoimmunity, and malignancy in mice, suggesting that regulation of SFK activity is essential for normal B cell function. The activity of SFK is regulated by phosphorylation of a conserved tyrosine residue at their C-termini by the ubiquitously expressed C-terminal Src kinase (Csk). Recently, we identified and characterized a novel transmembrane, lipid raft-associated Csk binding phosphoprotein (Cbp) and demonstrated its ability to mediate SFK inhibition by Csk. The overall goal of the experiments described in this application is to reveal the role of Csk and Cbp in regulation of B cell signaling and function. In particular, we will address whether and how Csk controls the threshold for B cell activation in vitro and in vivo. We will also study the role of Csk in autoreactive B cell responses to self-antigens. Our second aim is to investigate the mechanism of BCR mediated regulation of Cbp phosphorylation and its association with Csk in resting and activated B cells. Finally, using mice with conditional Cre mediated inactivation or modification of the cbp gene in B lineage cells, we will investigate the role of Cbp in regulation of lipid raft-associated SFK and B cell activation. Collectively, these studies will shed further light into the regulation of signal transduction in immune cells and provide data that may be useful in the treatment of diseases related to immune dysfunction.